X-ray intensifying screens are well-known in the prior art. Conventionally, a screen of this type comprises a support, an intensifying phosphor layer, and a topcoat or protective layer therefor. A reflective layer, such as a whitener (e.g. TiO.sub.2 dispersed in a suitable binder) may also be added into the screen structure. Commonly, this reflective layer is interposed between the phosphor layer and the support, or, alternatively, the whitener may be dispersed directly into the support. The reflective layer will maximize the light output of the intensifying screen during use. The protective layer is important since the phosphor layer contains the active ingredient used to expose an X-ray photographic film therewith and this phosphor is a very expensive ingredient. In operation, the intensifying screen absorbs X-rays that are impinged thereon and emits energy having a wavelength that is readily captured by the photographic, silver halide X-ray film associated therewith. Since the X-ray film elements are conventionally comprised of a support which is double-side coated with silver halide emulsions (e.g., a transparent support such as polyethylene terephthalate film having an emulsion layer on either side thereof), it is also conventional to employ two X-ray intensifying screens therewith, one facing each emulsion layer.
During use, the photographic silver halide film element is placed between the two X-ray intensifying screens with the emulsion layers held in intimate contact against the topcoat layers thereof. This intimate contact is important since it affects image quality. Thus, a book-type cassette is conventionally used to insure this intimate contact. The cassette containing the two screens and the duplitized X-ray film element is placed in proximity to a patient in the area under examination and the patient exposed to X-rays from a suitable source. After exposure, the film element is removed and processed to reveal the requisite image. Most of these steps must occur in the dark to protect the photosensitive film. Large hospitals, which handle many X-rays daily, generally use automatic changing equipment coupled with an automatic processing device in which the unexposed film is successively fed into position between a pair of X-ray screens automatically. This equipment may employ cassettes or specifically designed screen holders. If cassettes are used, film is interposed by a loader between the screens of a cassette which is then withdrawn from the loader and exposed with the patient and then returned to an automatic device which removes the film therefrom for automatic processing. The cassette with intensifying screens is re-cycled and coupled with more X-ray film, and so on.
These automatic devices are very handy in hospitals that employ successive operations of this type since all of the photosensitive elements are enclosed within the device and the operations can be handled under day-light conditions. As mentioned above, conventional X-ray intensifying screens have a protective topcoat that is intended to provide protection for the relatively expensive phosphor layer. The ideal topcoat possesses a number of desired properties including good adhesion to the phosphor-containing layer, abrasion and scratch resistance, among others. These properties are extremely important when the X-ray intensifying screen is designed to be used in the automatic changer systems, since these systems employ a harsher, physical environment for the screen. Thus, screens in this system can be easily damaged when the film drops in and out of the cassettes.
Other automatic film change systems exist that utilize self-contained cassettes loaded with multiple sheets of X-ray film. These machines operate in a rapid serial mode and are capable of moving the film from the feed cassette to the screen set (where the film is instantaneously compressed and the X-ray exposure made), and then to the unload cassette. All these operations can occur at a rate as fast as six film changes per second. These units can be easily jammed if the action of the film striking the X-ray screen edge or of the film sticking to the X-ray screen surface causes the topcoat of the X-ray screen to delaminate.
In Christini, U.S. Pat. No. 4,711,827, Dec. 8, 1987, there is described a novel topcoat layer which comprises a copolymer prepared from a mixture of approximately 5 to 50 weight percent acrylonitrile and 95 to 50 weight percent of styrene, for example. Although this topcoat material is tough, durable and resistant to static build-up, the adhesion between the topcoat and the phosphor layer is somewhat less than desirable. Since it is desirable to have the ultimate in topcoat adhesion to the phosphor-containing active layer, it has been a long-felt need in the industry to improve this quality and thus reduce artifacts that may be reproduced in the X-ray film element if portions of the topcoat are removed during use.
The prior art does teach the addition of magnesium sulfate or zinc sulfate to improve the performance of rare earth oxyhalide phosphors in X-ray image intensifying screens. There is no indication of an adhesion problem and the preferred levels of sulfate salt added as reported in this art are in excess of what is needed to promote adhesion between the active phosphor layer and the specifically defined topcoat protective layer as described in this invention.
It is an object of this invention to provide an X-ray intensifying screen that is suitable for use in automatic changers and the like. Another object is to provide such an intensifying screen having a topcoat layer with improved adhesion to the phosphor or active layer contained thereon. Still another object is to provide suitable adhesion without deleterious side effects.